Method for producing improved monocrystalline refractive material



United States Patent METHOD r012. PRGDUC'ING IMPROVED MON- cRYsTALLINE'REFRACTWE MATERIAL 5 Leon Merker, New York, N. Y.,' ass'ignor toNational Lead .Compan New York, N. Y., a corporation of New Jersey No'Drawing. Application February 2, 1953,

.Serial No. 334,709 1 Claim. or. 49-s9 refraction and dispersion:

One broad use of optical materials having high refractive indices andhigh'disp'e'rsion is in the manufacture of lenses and prism's. Forexample, materials having a high index of refraction are utilized in theconstruction of optical systems such as telescopic or high magnificationmicroscopic objectives. Furthermore combinations of optical materialswhich individually possess widely variant and the like. Highlyrefractive single crystal materialis also useful for the preparation'ofornamental objects such 40 as gem stones. Synthetic gems of varioustypes and colors are in great demand for both personal adornment andindustrial uses.

An object of this invention therefore is to provide a method forproducing a monocrystalline material which possesses exceedingly highindex of refraction with a high dispersion or relatively low 1! value. Afurther object is I and other objectswill become apparent from thefollow titanate in an oxidizing atmosphere at a high temperatureannealed crystal to an oxidizing atmosphere at temperature of about 200C. to about 500 C. to improve the color. It has been found that theinternal strains in the single crystal strontium titanate may berelieved by an- 5 nealing the crystal in an oxidizing atmosphere attemperature from about 1000 C. to about 1700 C. for 12 to 180 hours. Thecolor of the annealed crystal may be improved by subsequently treatingthe crystal in an oxidizing atmosphere at temperature from about 200 C.to about 7 500 C. for 6 hours to hours.

Copending application Serial No. 252,906 filed October 24, 1951, now S.Patent N0. 2,628,156, filed by .Leon

Merker and Langtry E. Lynd describes and claims in annealing themonocrystalline strontium titanate boule to obtain a substantiallystrain-free white boule. Copending application- Serial No. 252,906 isassigned to the same assignee as the instant invention.

The term strontium titanate is intended to embrace fussing particles ofstrontium titanate, it is necessary to employ high temperatures.Strontium titanate melts in the neighborhood of about 2050 C. and it istherefore preferred to use an oxygen-hydrogenfiame in order to obtainthe necessary temperatures without the possible introduction ofimpurities.

It is convenient to drop finely divided strontium crystal. Such aprocedure allows the crystal to build up upon itself graduallyincreasing in diameter until a boule or carrot-shaped single crystal ofstrontium titanate is formed.

It is preferred to employ a strontium titanate powdered ra'terial whichis substantially free from objectionable or incompatible impuritieswhich detrlmentally would affect the crystal structure. Startingmaterial should be finely divided and fairly uniform in size. Such amaterial may "be conveniently prepared by first reacting titaniumoxalate with strontium chloride to precipitate strontium titaniumoxalate. After thoroughly washing the strontium titanium oxalate, it isheated at a temperature of at least 500 C. to remove the oxalate portionthus forming strontium titanate. Preferably this strontium titanatematerial is then crushed and ground to obtain finely divided materialfrom which the strontium titanate single crystal is subsequently formed.

Using an oxygen-hydrogen burner and passing the finely divided strontiumtitanate feed material into the intensely heated zone, a single crystalboule of strontium titanate 1 inch long and /2 inch Wide was prepared in2 hours.

In order to produce an improved type of strontium titanate singlecrystal, the black boule was subjected to the oxidation treatmentcontemplated in the instant in vention. The black boule Was treated attemperature of 1500 C. for 25 hours and was then cooled slowly to 300 C.temperature and held at 300 C. temperature for 48 hours. The boule aftertreatment possessed a subdouble oxidation treatment.

The single crystal material should be subjected first to a very hightemperature oxidation treatment, i. e. from 3 temperatures of about 1000C. to about l700 C. These high temperature treatments should bemaintained between 12 and 180 hours. At this stage the monocrystallinestrontium titanate possesses a purplish or pinkish appearance instead ofthe former black color. The monocrystalline material is then subjectedto the second oxidation treatment which should be carried out attemperature from about 200 C. to about 500 C. for 6 hours to This lowtemperature treatment is necessary in order to obtain a substantiallyWhite boule. The two oxidation treatments can be carried out in separatefun naces or in the same furnace by merely lowering the temperatureafter the first treatment to that required for the second. When themonocrystalline strontium titanate material is processed according tothe instant invention the strains in the crystal are relieved and bouleswhich are properly annealed are obtained. These monocrystalline titanateare substantially white in masses of strontium color and are properlyannealed and may be shaped into many bodies useful for optical purposessuch as, e. g.

cut into lens blanks suitable for preparation of lenses, prisms, andother optical products; and also may be cut into various shapes andsizes to produce many articles such as ornamental objects and noveltygems, Which depend for their attractiveness and usefulness on the highrefractive index of the material of which they are composed.

The index of refraction of the strontium titanate crystal is extremelyhigh, i. e. about 2.4, and the reciprocal dispersion is about 13. Thesevalves will vary slightly as various types of starting materials andprocessing conditions are employed.

It should be noted that the strontium titanate crystal when cut into alens has a high magnifying power and a short focal length. Such materialis particularly useful for telescopic and microscopic objectives and thelike, achromatic lenses and prisms and for other optical purposes whichadvantageously utilize wide fields, high apertures and short focallengths. It should be noted that the index of refraction and reciprocaldisperson are very different from any type of known glass.

This monocrystalline mass of strontium titanate after A proper annealingaccording to this invention will not tend to chip, splinter or crack aseasily as those crystals which are not properly annealed.

While this invention has been described and illustrated by the examplesshown, it is not intended to be strictly limited thereto and othermodifications and variations may be employed Within the scope of thefollowing claim.

I claim:

Method for producing an improved single crystal of strontium titanateprepared by co-fusion of finely divided strontium titanate particleswhich comprises annealing the single crystal of strontium titanate in anoxidizing atmosphere at temperatures from about 1000 C. to about 1700 C.for 12 to 180 hours to relieve the strains in said crystal andsubsequently heating the crystal in an oxidizing atmosphere attemperatures from about 200 C. to about 500 C. for 6 to 80 hours toimprove the color.

References Cited in the file of this patent UNITED STATES PATENTS988,230 Verneuil Mar. 28, 1911 1,004,505 Verneuil Sept. 26, 19111,298,540 Miller Mar. 25, 1919 1,436,164 Goldschmidt Nov. 21, 19222,507,253 Howatt May 9, 1950 2,610,129 Eversole et al. Sept. 9, 19522,628,156 Merker et al. Feb. 10, 1953 2,634,554 Barnes Apr. 14, 1953FOREIGN PATENTS 243,251 Great Britain Nov. 26, 1925 252,698 SwitzerlandOct. 1, 1948 608,453 Great Britain Sept. 15, 1948 662,782 France Nov.26, 1927 664,889 Great Britain Ian. 16, 1952 OTHER REFERENCES G. E. C.Journal, August 1944, pages 56-69. Kremers: Synthetic Optical Crystals,Ind. and Eng. Chem, November 1940, vol. 32, pages 14784482.

